When using the SharePoint Migration Tool to move to 365, you may encounter some cryptic errors in the tool’s error reports, such as the following:

(In this example I was trying to move a discussion board from SP 2013 on-prem)

 

Source Destination Item name Extension Item size Content type Status Result category Message Error code 

https://sp.contoso.com/Lists/Discussions/TestThread https://contoso.sharepoint.com/Lists/Discussions/TestThread TestThread 0 Folder Failed SERVER FAILURE Errors or timeout for Server Processing the file:Not all the items in the package have been migrated 0x01710009…………

https://sp.contoso.com/Lists/Discussions/TestThread/2_.000 https://contoso.sharepoint.com/Lists/Discussions/TestThread/2_.000 2_.000 0 0 Item Failed SERVER FAILURE Errors or timeout for Server Processing the file:Not all the items in the package have been migrated 0x01710009………………

 

In my case, the solution was to look into the “Details” folder of the Migration Tool report, and look at the UserNotMapped_R1.csv file.  This contained a list of SIDs for users for our on-prem installation, that while their accounts where in SharePoint, they did not exist in our AD.  Basically the Migration Tool errors out when it encounters a problem mapping an account to 365 SharePoint.

The solution was to look them up from the on-prem installation, and then delete them:

PS C:\Users\SPFarm> $u = Get-SPUser -Web https://sp.contoso.com -Limit ALL |where {$_.Sid -eq "s-1-5....."}
PS C:\Users\SPFarm> $u

UserLogin DisplayName
--------- -----------
CONTOSO\jdoe1  John Doe

Then, once the user account was known, visit https://sp.contoso.com/_catalogs/users and create a custom view to filter on the “UserLogin” listed above, with an option to Edit.  If you click on the “Edit” next to their account, you will have an option to delete them from the site collection.  I did this for all the SIDs provided in the file, and it resolved the issue.